<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* 实现思路：
        利用border-radius 拿到width,height的一半，设置横向和纵向半径
        */
        .box {
                border-bottom: 100px solid red;
                border-left: 50px solid transparent;
                border-right: 50px solid transparent;
                height: 0;
                width: 100px;
            }
    </style>
</head>
<body>
    <div class="box"></div>
</body>
</html>